This invention relates to an improvement of a scanning projection light source for a barcode device, and particularly to an improvement that is capable of generating projection light which has arched tangent sides for forming a desirable light source for scanning a barcode.
A good projection light source is essential for obtaining a desirable scanning result. The ideal projection light source condition is generally formed in a circular or elliptical manner. As most barcodes are printed in an elongated form, hence theoretically, the ideal scanning light spot for reading a barcode is square or rectangular. Elliptical is less desirable, and circular is even less desirable than elliptical. When using a circular or near-circular light spot to scan a barcode, the reflecting signal is close to a sinusoidal wave, which is not close to a digital signal. The circular light spot is relatively undesirable in the condition of reading the barcode by length and width dimension ratio. However, when scanning the barcode using a square or rectangular light spot, the reflecting signal is close to a square wave and is close to a digital signal, and therefore is more desirable for reading the barcode.
Another condition for reading a barcode within a given range is to make the projecting light parallel, and the size of the light spot should be suitable for reading the barcode. Hence most scanning type barcode readers now being used employ a laser light source. In order to make the product size compact, most of the products also use a laser diode (as shown in FIGS. 1 and 2). The following is an example of the construction.
The laser diode 1 is set to project out an elliptical light of 30 degrees in the X axis and 10 degrees in the Y the axis. In order to reach greater reading distance, a converging lens 2 is disposed to convert the light to become as parallel a light beam as possible. As mentioned before, the elliptical light source is not the most desirable light source, but the laser diode 1 projects light in an ellipse form. Hence to design and make an aspherical lens to match the elliptical light of the laser diode 1 is a very difficult and costly work. The resulting light spot has a large size and is not suitable for reading a barcode.
The outgoing light beam from the converging lens 2 is circular and almost parallel. But its size is too large. The light beam can be narrowed by projecting the output light beam through a round aperture 61 of a first photo mask 6 disposed in front of the lens 2. The resulting smaller light spot then may be deployed to project on an object (barcode).
Conventional photo masks now being used mostly have square or rectangular apertures. They have two flaws. One is that when a light beam passes through the narrow aperture, a photo grating effect will be generated. Another one is that an aperture that is too narrow will result in light beam distortion at a distance. These problems have been confirmed in experiments.
For resolving the aforesaid problems, there is a proposal which uses a cylindrical lens to generate a desirable narrow projecting light source. While it may pass through the narrow aperture without resulting in light spot distortion, it still cannot generate desirable resolution. Moreover, producing a glass cylindrical lens still has technical problems.
In view of aforesaid disadvantages, it is therefore an object of this invention to provide an improvement that uses two photo masks in front of a converging lens of a laser light source for producing the desirable results. According to this invention, the first photo mask is substantially constructed like the conventional one. In front of the first photo mask, a second photo mask is disposed which has a second aperture with two corresponding and curve-shaped light blocking side tabs. The laser light spot emitted by the laser diode projects on the converging lens to become a parallel outgoing light beam. The outgoing parallel light beam then passes through the round aperture of the first photo mask to become a small and round light spot. The round light spot then passes through the second aperture to form an output light spot which has arched tangent sides. The arched tangent side is approximate to a straight line at a close distance, but forms a clearer arched tangent side at a longer distance. Hence it may result in a higher resolution within a selected distance and may be used for scanning relatively a lower resolution barcode at a longer distance.
Another object of this invention is to provide four light blocking side tabs arranged in an up and down, and left and right, manner around the round aperture.
A still further object of this invention is that the tabs will cover approximately ⅙ to xe2x85x9 of the area of the round aperture of the first photo mask.